PGG Wrightson has admitted to breaching biosecurity protocols, after seeds of a prohibited grass blew off a truck in prime arable farmland in Canterbury.
A small amount of black grass seed - described as an "egg cup full" by the Ministry for Primary Industries - blew off one of its trucks along a 40km stretch of State Highway 77 from Ashburton to Methven.
Under MPI regulations, all transported seed that has not been officially released must be kept inside a covered vehicle.
PGG Wrightson Seeds group general manager John McKenzie says the seed was in open steel bins inside the truck, allowing it to blow into the air.
Mr McKenzie says the company will assist farmers so they can recognise what black grass looks like as well as help with surveillance.
He says the ministry will decide later this month whether it will prosecute the company.
The Foundation for Arable Research says it will be hard to locate the seed but it is confident of finding any plants.
Chief executive Nick Pyke says the seed could remain viable for a number of years.
He says black grass is visible from November to April and significant surveillance over time will be required to eradicate it.
"It is going to be hard to find those few that may establish but because it is distinctive we're pretty confident that with good surveillance methodology and skilled people looking for it that we'll be able to pick them up."
Mr Pyke says there have been two reported cases of black grass in New Zealand but it did not become established.
MPI says the seeds are a low biosecurity risk and not many are likely to germinate because they are immature and most landed on barren ground.
But Federated Farmers disagrees, saying the ministry is downplaying the issue and the comments are unhelpful.